LIFE HISTORIES OF CERTAIN FRESH-WATER FISHES. 481 
almost imperceptibly. When first hatched the heart is in 
a perpendicular position and the blood is colourless, but after 
a few days the heart becomes horizontal and the blood red. 
The growth and development of the young fish in the 
aquarium was fairly rapid. Their food from the first appar- 
ently consisted entirely of Entomostraca, and they attacked 
and successfully engulfed fairly large daphnids. When 
9 days old their length was from n to 13 mm., and in 21 days 
they had increased to from 16.0 to 19.0 mm., and they were 
by that time easily recognisable as Pike, having developed 
the large jaws and peculiar shaped head of the adult. At 
this stage also the paired ventral fins are visible in a rudiment- 
ary condition, and the rays of the dorsal and anal fins are in 
process of development. The tail also is in the heterocercal 
stage, the two sets of 8 rays representing the upper and lower 
halves of the adult fin being clearly recognisable below the 
upturned notochord. The larval fins are still present but 
are very ragged in appearance as if they were fraying off. 
At the age of about 6 weeks the process of development is 
almost complete, the fish being about 30 mm. (i£"-i£") long. 
Owing to their immense appetites it was not considered 
advisable to keep these fish longer in the aquarium. In one 
experiment, however, two were retained until the middle of 
August when they were about 5 months old, and they had 
then attained a length of about 3 inches (75.0 to 85.0 nnn.). 
Judging from the fact that a number of young Pike caught 
in August and September measured from 75.0 to 95.0 mm. 
in length, the development of the aquarium specimens would 
appear to have been about normal. 
As to the rate of growth beyond the first season almost 
nothing is known. We have tried the experiment of marking 
Pike with silver labels and releasing them in the hope of their 
being recaptured ; but, although we have so marked between 
30 and 40 fish of various sizes and have advertised the fact 
throughout the district, none of these fish have been recovered, 
so far as I know, except five which we have retaken ourselves 
in the Sutton Broad. We cannot, of course, learn much as 
to the growth or migrations of the Pike from that amount of 
material, but it is interesting to notice that all five fish were 
